mm: add comments to explain mm_struct fields

	unsigned long hiwater_rss;	/* High-watermark of RSS usage */

	unsigned long hiwater_vm;	/* High-water virtual memory usage */

	unsigned long total_vm, locked_vm, pinned_vm, shared_vm, exec_vm;

	unsigned long stack_vm, reserved_vm, def_flags, nr_ptes;

	unsigned long total_vm;		/* Total pages mapped */

	unsigned long locked_vm;	/* Pages that have PG_mlocked set */

	unsigned long pinned_vm;	/* Refcount permanently increased */

	unsigned long shared_vm;	/* Shared pages (files) */

	unsigned long exec_vm;		/* VM_EXEC & ~VM_WRITE */

	unsigned long stack_vm;		/* VM_GROWSUP/DOWN */

	unsigned long reserved_vm;	/* VM_RESERVED|VM_IO pages */

	unsigned long def_flags;

	unsigned long nr_ptes;		/* Page table pages */

	unsigned long start_code, end_code, start_data, end_data;

	unsigned long start_brk, brk, start_stack;

	unsigned long arg_start, arg_end, env_start, env_end;